Foreign tourists walk on the pavement along the Imperial Palace Gardens in the intense heat in Tokyo on July 12, 2023. AFP
National broadcaster NHK warned viewers that the heat was at life-threatening levels, as temperatures soared to nearly 40 degrees Celsius (104 degrees Fahrenheit) in some places, including the capital Tokyo.
"Please stay hydrated and use air conditioners appropriately, and refrain from outings that seem difficult," a news presenter said.
The government issued heatstroke alerts for 20 of the country's 47 prefectures, mainly in the east and southwest, affecting tens of millions of people.
Heat can kill by inducing heatstroke, which damages the brain, kidneys and other organs, but it can also trigger other conditions such as a heart attack or breathing problems.
Kiryu city in Gunma prefecture, north of Tokyo, saw the mercury reach 39.7C while Hachioji in western Tokyo reached 38.9C, according to the Japan Meteorological Agency.
Japan's highest temperature ever recorded was 41.1C, which was first recorded in Kumagaya city, in Saitama, in 2018 and then matched in Hamamatsu city, Shizuoka, in 2020.
Some places experienced their highest temperatures in more than four decades on Sunday, including Hirono town in Fukushima prefecture with 37.3C, and hot spring resort city Nasushiobara with 35.4C, according to the weather agency's data.
Meanwhile, torrential rain continued to lash northern Japan, where flooding and at least one landslide have been recorded.
A man was found dead in a car submerged in a rice field in Akita prefecture, police told AFP, a week after seven people were killed in similar weather in the country's southwest.
Since last weekend, a heavy band of precipitation has dumped record-breaking amounts of rain in some parts of Japan, causing rivers to overflow and sodden earth to collapse in landslides.
Japan is experiencing its annual rainy season, which often brings heavy downpours, and sometimes results in flooding and landslides, as well as casualties.
But scientists say climate change is intensifying the risk of heavy rains in Japan and elsewhere because a warmer atmosphere holds more water.